1. Field of the Invention
This invention relates to an electrical self-purification ignition plug.
2. Description of the Prior Art
Concerning the prevention of fouling of an ignition plug, a technique of burning off or blowing away carbon or the like deposited on a porcelain insulator ignition leg and composed of a combustion residue adhered thereto with an electrical energy of a spark discharge has heretofore been well known as an electrical self-purification action.
In order to cause an ignition plug provided with an air gap between a grounded electrode and a center electrode to display the above mentioned self-purification action, an attempt has been made to locate the front end of the center electrode at a position which is recessed into the end surface of the porcelain insulator ignition leg and make the outer diameter of the front end of the center electrode smaller than the inner diameter of the ignition leg. In this case, if the diameter of the front end of the center electrode is too small, the life of the ignition plug becomes short. As a result, it is desirous to make the inner diameter of the ignition leg large. But the outer diameter of the ignition leg is restricted by the presence of an annular gap between the ignition leg and the inner surface of the metal fitting of the ignition plug proper, that is, the presence of a so-called gas volume. As a result, the ignition leg must be thin in thickness and hence the ignition leg having a thin thickness is exposed to the high temperature combustion gas and tends to produce a hot spot thereon, thereby inducing a preignition failure.